Technical Field
The present invention relates to a display device and a driving method thereof.
Discussion of the Related Art
An active matrix-type electroluminescent display device includes a self-emitting Organic Light Emitting Diode (OLED), and has advantages of a fast response time, a high light emitting efficiency, high luminance, and a wide viewing angle.
An OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The electroluminescent display device includes pixels each including an OLED, wherein the pixels are arranged in matrix, and adjusts luminance of the pixels in accordance with a gray level of video data. Each of the pixels includes a Thin Film Transistor (TFT) that controls a driving current flowing in an OLED in accordance with a voltage applied to a gate electrode and a source electrode of a corresponding pixel. In addition, a gray level (luminance) of each pixel is adjusted dependent upon an amount of light emitted by the OLED, which is proportional to the driving current.
FIG. 2 illustrates a circuit of an existing pixel, and FIG. 3 illustrates a driving waveform of FIG. 2.
Referring to FIGS. 2 and 3, a pixel PXL includes an Organic Light Emitting Diode (OLED), a plurality of Thin Film Transistors ST1 to ST3 DT, and two capacitors Cst1 and Cst2. In FIG. 2, “Coled” indicates a parasitic capacitance of the OLED.
The TFTs ST1 to ST2 DT are implemented as an n-type MOSFET (which is hereinafter referred to as an NMOS). In addition, for a low-speed driving, a first switch TFT ST1 is in the form of an NMOS-type oxide TFT having an excellent off-current characteristic, and other TFTs ST2 and ST3 DT are in the form of an NMOS-type LTPS TFT having an excellent response characteristic.
The pixel PXL is driven during a scanning period and an emission time Tem. The scanning period may be set as approximately one horizontal period 1H, and includes an initialization time Ti, a sampling time Ts, and a programming time Tw.
During the initialization time Ti, a predetermined reference voltage Vref is supplied to a data line DL. During the initialization time Ti, a voltage of the gate node Ng is initialized to the reference voltage Vref, and a voltage of a source node Ng is initialized to a predetermined initialization voltage Vinit.
During the sampling time Ts, an electric potential of the gate node Ng is maintained at the reference voltage Vref, but an electric potential of the source node Ns is increased by a drain-source current Ids. In this source-follower method, a gate-source voltage Vgs of the driving TFT DT is sampled to a threshold voltage Vth of the driving TFT DT, and the sampled threshold voltage Vth is stored in the first capacitor Cst1. At a time when the sampling time Ts is finished, a voltage of the gate node Ng becomes the reference voltage Vref and a voltage of the source node Ns becomes a voltage that corresponds to a difference between the reference voltage Vref and the threshold voltage Vth.
During the programming time Tw, the data voltage Vdata is applied to the gate node Ng, and a result of distribution of a voltage variation Vdata-Vref of the gate node Ng between the capacitors Cst1 and Cst2 is reflected to the source node Ns. Thus, the gate-source voltage Vgs of the driving TFT DT which corresponds to a desired driving current is programmed.
During the emission time Tem, the OLED emits light in accordance with the driving current to thereby realize luminance corresponding to image data.
In an existing display device, the programming time Tw is determined by a driving frequency. If the programming time Tw is determined, the emission time Tem is fixed as well.
FIG. 4 is a diagram illustrating a duty control method of the existing display device.
Referring to FIG. 4, a frame period of a display device is determined by a driving frequency, and accordingly, a programming time and an emission duty are fixed as well by the driving frequency.
In the course of inputting frame data, a period where data is not input between a previous frame data and a next frame data is a blank time BT. During the blank time BT, various control procedures may be performed to measure luminance or current of a pixel and improve degradation of image quality.
However, an emission duty is fixed by a programming time Tw, which is determined by a driving frequency, in the existing display device, and thus, there is a limitation in securing a blank time when resolution are enhanced. As a result, there is insufficient time for performing functions required to be done during the blank time, such as sensing luminance or current and improving image quality.